Voter registration is underway in Africa's most populous nation as Nigerians prepare for April elections.

More than 70 million Nigerians are eligible to vote in this presidential election, and campaigning is well underway with all of the major parties having chosen their candidates.

President Goodluck Jonathan is the nominee of the ruling People's Democratic Party, which has won the past three presidential elections.

"With the present constitution, no party in Nigeria has the capacity to undo PDP," said Delta State University Political Science Professor Isitoah Ozoemene, who says Jonathan is the clear frontrunner, just eight months after taking charge of the party following the death of President Umaru Musa Yar'Adua. "Goodluck has been quiet. He has been listening, and he has subtly been able to exact control. He does not shout," he added.

Nigeria's president Jonathan addresses delegates during the primaries of the ruling People's Democratic Party in Abuja, 13 Jan 2011

Jonathan broke an informal power-sharing agreement in the ruling party that rotated the presidency between north and south. That deal said a northerner should be the nominee for this vote to finish out what would have been President Yar'Adua's second term.

But in the ruling party primary, Jonathan handily defeated the northern candidate, former vice president Atiku Abubakar.

That opens the way for northern candidates from other parties, chief among them former military ruler Muhammadu Buhari. Official Abubakar Sadiq Abba, of Buhari's party, says Abubakar's loss in the ruling PDP primary is a big gain for Buhari.

"I believe the majority of northerners will vote Buhari from even within the PDP family, because they will use it as a protest vote to teach their governors a big lesson for betraying their own trust and confidence reposed in them during the last primary elections," he said.

Abba says Mr. Buhari is well positioned to win a majority of northern states.

"He is already far ahead of everybody, including President Jonathan in northern Nigeria," said Abba. "So 15 of the 19 northern state are already in Buhari's hands as far as the followers are concerned, as far as the citizens are concerned, and as far as the voters are concerned."

Political science professor Ozoemene believes the preponderance of northern candidates will split the northern vote and weaken Mr. Buhari's chances.

"He is a northern leader who is banking on the support of the entire north," he said. "But for me the north is now fragmented. There used to be a time when it was one north, one people, one destiny. Today the scenario has changed."

Among the challengers for the northern vote is the Action Congress of Nigeria candidate, former anti-corruption chief Nuhu Ribadu. Ribadu supporter Babafe Ojudu says the party's consensus shows it has reach beyond the north.

"It shows the spirit that brought all of us together it shows that in our party we can talk to one another we can reason with one another we can compromise with one another and reach a consensus that's the beauty of democracy, that's the beauty of ACN party," said Ojudu.

Ribadu supporter Yomi Adedogu says the ruling party is mistaken if it believes it is the only party with national appeal.

"Jonathan has won his party ticket," he said. "Ribadu has won my party ticket. And I can assure you that we are the only party any party can not beat in this country."

If no candidate wins 24 of Nigeria's 36 states in the April 9th vote, there will be a second-round run off between the top two finishers one week later.